1. Field of the Invention
This invention relates to a silicone rubber molded product in which a portion of silicone rubber contains 5 to 75 wt % carbon black and a portion of silicone rubber contains substantially no carbon black are united together. This invention also relates to a method for producing this silicone rubber molded product.
2. Description of the Prior Art
Silicone rubber molded products in which the cured body has a portion of silicone rubber containing carbon black and a portion of silicone rubber containing substantially no carbon black has been widely used as two-color keyboard pads, zebra-type connectors, and electrical joints for various types of electrical appliances.
An improved method for producing this type of silicone rubber molded product was disclosed in Japanese Patent Publication No. Sho 56 [1981]-41417, published Sept. 28, 1981, in the names of Sado et al. In this method, an organic perioxide is added to unvulcanized silicone rubber containing carbon black and a conductive silicone rubber cured body is formed by hot compression molding. Subsequently, an organic peroxide is also added to an insulating unvulcanized silicone rubber containing substantially no carbon black and molded along with the above-mentioned cured body by hot compression molding. As a result, a silicone rubber molded product is formed in which a portion of the conductive silicone rubber cured body are united together.
However, because carbon black included in the conductive silicone rubber tends to cause vulcanization inhibition with the organic peroxide, the applicable organic peroxides are limited to dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane. When these organic peroxides which are less likely to be influenced by vulcanization inhibition with carbon black, are used, the following drawbacks occur. Although the mold temperature is set at a high temperature (i.e. 150.degree. C. or higher, or 170.degree. C. or higher), molding must be continued for 3 to 4 minutes or 10 minutes or more, resulting in increased consumption of electrical power and poor production efficiency. The surface of silicone rubber results in curing inhibition due to the air trapped in the gap between the mold and silicone rubber. Moreover, because foaming occurs when hot air vulcanization is carried out, the method of vulcanization is limited.
Ehrreich and Reti in U.S. Pat. No. 3,620,873, issued Nov. 16, 1971, describe making plastic members of electrically conductive or thermally conductive sheets or strips and non-conductive areas.